Nonlinear positive gradient force mechanism

ABSTRACT

A mechanical linkage interposed between the armature and opening spring for an electromagnet cooperates with the spring to provide a nonlinear positive spring gradient component acting on the armature, which gradient component closely follows the flux force gradient generated by the minimum current required for pulling in the armature. Very near the end of the armature stroke the flux force gradient diverges from the spring gradient component to provide extra force which overcomes contact pressure spring forces.

United States Patent Inventor Howard R. Shaffer Glenslde, Pa.

Appl. No. 9,658

Filed Feb. 9, 1970 Patented Nov. 30, 1971 Assignee ITE Imperial Corporation Philadelphia, Pa.

NONLINEAR POSITIVE GRADIENT FORCE MECHANISM 5 Claims, 8 Drawing Figs.

US. Cl 335/172, 3 35/239 Int. Cl. "01h 7/08 Field of Search 335/172,

192, l9l, 176, 239, 240, 42, 35; ZOO/153.7

References Cited UNITED STATES PATENTS 2,486,596 11/1949 Graves 335/239 2,904,730 9/1959 Peek 335/239 3,391,36l 7/1968 Jencks etal. 335/176 Primary ExaminerHarold Broome Anorney0strolenk, Faber, Gerb & Sotfen wx lll m ww I PATENTED unvao 197:

SHEET 3 BF 5 PWMT NONLINEAR POSITIVE GRADIENT FORCE MECHANISM This invention relates to electromagnets in general and more particularly relates to an electromagnet in which the opening spring gradient closely follows the flux gradient during the closing stroke of the armature.

Difficulties have been encountered in providing long time delays by utilizing electromagnetic time-delayed structures having dashpots. In circuit breaker trip units this difficulty results in a limited time change for a given current change.

These difficulties are due to the fact that the armature retarding force provided by mechanical springs is linear, while the magnetic closing force is nonlinear and increases at a much faster rate than the retarding force as the armature closes. Thus, on closing of the armature the resultant force acting thereon increases as the armature closes. This increasing resultant force overcomes the dashpot retardation so that the armature moves faster and the time delay is reduced.

In accordance with the instant invention, armature speed on closing is reduced to provide a longer time delay by having the armature retarding force increase in the same proportion as the magnetic force so that the resultant armature moving force remains constant throughout most of the stroke. To provide an additional tripping force near the end of the closing stroke, the magnetic closing force and the spring retarding force are made to diverge near the end of the closing stroke.

In accordance with the instant invention, a nonlinear positive gradient force mechanism is constructed with springs acting on the movable armature through linkages. By maintaining the difference between the armature retarding force and the magnetic pull'in force very close to the same value throughout the closing stroke, a longer time delay results.

Accordingly, a primary object of the instant invention is to provide an electromagnet in which the opening spring acts through levers to produce a nonlinear positive spring gradient force component in opposition to the magnetic gradient closing force.

Another object is to provide an electromagnetic structure of this type in which the spring gradient and flux gradient closely follow one another.

Still another object is to provide an electromagnetic device of this type in which thereis a dashpot for providing a time delay.

A further object is to provide a lever and spring system which produces an output force that has an effective nonlinear positive gradient.

These objects as well as other objects of this invention will become readily apparent after reading the following description of the accompanying drawings in which:

FIG. 1 is a longitudinal cross section of a circuit breaker having a magnetic trip unit constructed in accordance with teachings of the instant invention.

FIGS. 1A and B are enlarged end views of the magnetic trip mechanism of FIG. I. In FIG. 1A the magnet is deenergized, while in FIG. 1B the magnet is energized and the armature is in its fully pulled-in position.

FIGS. 2, 3, 4 and 5 are similar to FIG. IA and illustrate different embodiments of the instant invention.

FIG. 6 is a graph showing the relationship between the magnet and spring force gradients.

Now referring to the figures, and particularly FIGS. 1, 1A and 13. Circuit breaker 10, incorporating magnetic trip unit 100 constructed in accordance with teachings of the instant invention, is typically a commercially available three-phase molded case unit, only one pole of which is illustrated. Circuit breaker 10 includes molded base 11, main cover 17, end covers 18 and shields 19 located at the line and load ends of circuit breaker 10.

The current path between the line terminal strap 20 and load terminal strap 21 for each pole proceeds from stationary contacts 22, 23 to movable contacts 24, 25 carried by contact arms 26, 27 through flexible braid 28, contact carrier strap 29 and trip unit strap I01. Trip unit 80 is a modification of a trip unit illustrated in US. Pat. No. 3,3l9,l95, issued May 9, 1967, to A. Strobel and .l. C. Brumfteld for a Circuit Breaker Trip Unit Assembly.

Trip unit includes vertically movable actuating rod 113 having threaded adjustable extension 57 engageable with tripper bar extension member 58. Engagement of extension 58 by extension 57 pivots tripper bar 53 in a counterclockwise direction, with respect to FIG. 1, to release auxiliary latch 56. In a manner well known to the art, the release of auxiliary latch 56 releases contact operating mechanism 32 to pivot tie bar 33 in a counterclockwise direction, which in turn moves contact arms 26, 27 in a counterclockwise direction to the position illustrated in FIG. 1.

Thermal tripping is achieved by bimetal strip 54 fixed at its lower end to the leftmost strap 29. Upon heating of bimetal 54, the upper end thereof deflects to the left engaging adjusting screw 55 extending from tripper bar 53 and pivoting bar 53 in a counterclockwise direction.

The instantaneous or magnetic trip portion (FIGS. IA and 1B) of trip unit 80 includes strap 101, which constitutes the energizing term for the magnetic frame consisting of generally U-shaped yoke I02 and flat movable armature 103 secured to the horizontal leg of bracket 10S. Screw 78 engaging threaded insert 77 fixedly secures yoke 102 to base 11. Rivets I06 pivotally secure the lower ends of links 107a and 1070 to the vertical leg of bracket 105. Rivets I08 pivotally secure the upper ends of links 107a, l07c to the lower ends of links 107b, 107d, respectively, and rivets III pivotally secure the upper ends of links 107b, 107d to the vertical leg of bracket 112. Rivets 99 secure Lshaped bracket 112 to U shaped bracket 98. The latter is fixedly secured by rivets 97 to the molded housing part 96 of trip unit 80. Brackets 79 connect yoke 102 to housing part 96.

Actuating rod 113 is secured to armature 103 and extends upwardly therefrom through a guide aperture in the horizontal leg of bracket 112. Overlapping rods 109 extend in opposite directions from pivots 108 to form a collapsible arm about which coiled compression spring is wound.

In the absence of current flow through conductor I01, armature 103 is held in the raised position of FIG. 1A, spaced from yoke 102 by air gaps 104. When current in conductor 101 exceeds the pickup value (minimum current value required to move armature I03 toward yoke 102), the magnetic force generated by flux across the sections of air gap 104 starts to move armature I03 toward yoke 102. As armature I03 moves toward yoke 102, the retarding force exerted by spring 110 coupled to armature 103 through links l07a-d increases in a nonlinear manner and follows a curve similar to the magnetic force curve, so that the resulting moving force remains essentially constant as armature I03 moves toward yoke I02. Thus, the pickup point of magnet I02, I03 will be the same for all armature positions and the dropout point will be slightly less than the pickup point. The parameters for links I07a-d and spring 110 as well as the locations of pivots I06, 1 11 are chosen so that the spring force gradient diverges from the flux force gradient at the point where actuating rod extension 57 engages tripper bar extension 58 so that an additional force is available for tripping.

FIG. 6 shows typical magnetic curves A and C and divergent spring force curve B of the instant invention. Comparing curves A and B, it is seen that the net forceAF available for movement of armature I03 remains practically the same for air gaps 104 from widths of 0.25 inches to 0.l2 inches and then increases from 0.l2 inches to 0.0l inches. Note how a typical spring gradient D diverges very rapidly from flux gradient A.

The linkages of FIGS. 2 and 3 are similar in operation to the linkage of FIGS. IA and 13 except that in FIGS. 2 and 3 the parts are reduced in number and rearranged to use tension springs 210, 310. In particular, end 211 of tension spring 210 (FIG. 2) is anchored to bracket 212, and the other end 209 of spring 210 is anchored to the free end of the shorter arm 207a of bell crank or lever 207. The latter is connected, at a point intermediate its ends, to fixed bracket 214 by pivot pin 208. The free end of the longer arm 207b of crank 207 is slidably connected to bracket 205 on the upper surface of armature 103 by pin 206 extending into slot 215 of bracket 205. Slot 215 extends generally perpendicular to the direction of movement for trip rod 113.

In the embodiment of FIG. 3, the free end 309 of tension spring 310 pulls downward to bias bellcrank 317 in a clockwise direction about its pivot 308 at fixed bracket 314. The other end 311 of spring 310 is fixed to stationary bracket 312. The end of bellcrank 307 remote from end 309 is slidably connected to bracket 305 at the top of armature 103 by pin 306 which extends into slot 315 in bracket 305. The embodiment of Fig. 4 is similar to the embodiment of FIG. 3, with the addition of dashpot 418 connected by link 417 to crank 307 at end 309 thereof. Now instead of armature 103 moving instantaneously, as in the embodiment of FIG. 3, there will be a time delay provided by dashpot 418, and this time delay will be greater than for a time-delayed magnetic structure of the prior art.

This is explained by reference to FIG. 6, in which the solid magnetic curve C would be the curve just at pickup. Assuming an increase in current over the pickup current to a value where the magnetic curve is curve A, the force available to move armature 103 and dashpot 418 is AF, at an air gap of 0.25 inches, whether the retarding force is the nonlinear positive gradient force B or the linear spring force D. If the typical spring force curve D is used, the available force to move armature 103 becomes larger and larger as air gap 104 becomes smaller, until AF is reached at an air gap of 0.06 inches. Therefore, the speed of armature 103 increases as air gap 104 decreases and the time delay is short. Now considering the nonlinear positive gradient force curve B, the available force to move armature 103 remains practically constant until AF is reached, and AF is only slightly larger than AF Therefore, the speed of armature 103 remains practically constant through this strokefrom an air gap of 0.25 inches to 0.06 inches. Note that for the example shown the ratio of AF to AF, is 2% and the ratio ofAF to AF is 7.

The embodiment of FIG. 5 is similar to that of FIG. 4 except that another form of electromagnet is illustrated. In FIG. 5 dashpot 518 is connected to armature 503 by links 517, 521, joined at pivot 520 near one end of armature 503. The other end of armature 503 is engageable with pivotally mounted circuit breaker tripper element 513. Armature 503 is pivotally mounted at 514 to one end of bracket 512. The other end of bracket 512 is connected to end 511 of tension spring 510, whose other end 509 is connected to one end of crank or lever 507 pivoted at 508 to bracket 512 secured to yoke 502. The other end of crank 507 is connected to armature 503 at the end thereof adjacent to trip element 513, by pin 506 extending into longitudinally extending slot 523 of armature 503. Multiturn conductor 501 is wound about one leg of U-shaped yoke 502, mounted in fixed position on bracket 522.

When current through conductor 501 exceeds the pickup value, armature 503 pivots in a counterclockwise direction about pivot 514, with this movement being retarded by dashpot 518. By the time air gap 504 has diminished to a very low value, armature 503 has engaged and moved tripper element 513 to release the contact operating mechanism of the circuit breaker.

Accordingly, it is seen that this invention provides means for obtaining a longer time delay in the operation of electromagnetic mechanism that are used in circuit breakers. This increased time delay is obtained by utilizing a linkage that results in a nonlinear positive gradient output force for retarding armature movement.

That is, the ratio of the change of spring force for a given change of movement is defined as a spring constant. For a typical spring this is the slope of curve D of FIG. 6. A typical spring has a constant slope but the nonlinear positive gradient force has a varied slope which increases as the air gap decreases. Therefore, for given increments of load, the movement increment decreases. In the example of FIG. 6, a change of 0.5 pounds from 0.25 inches air gap results in a movement to 0.15 inches, or a change of 0.5 pounds is accompanied by a movement of 0.10 inches. The addition of another 0.5 pounds causes the armature to move to the 0.105 inches position. For this latter force change of 0.5 pounds, the movement is 0.045 inches. Therefore, this invention provides means for changing the efi'ective spring constant depending on load.

Although there have been described preferred embodiments of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited not by the specific disclosure herein but only by the appending claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. An electromagnetically operated device including a magnetic frame and a conductor means coupled thereto so that electric current flow in said conductor means generates magnetic flux in said frame; said frame including a relatively stationary yoke and a relatively movable armature, linkage means, and spring means connected through said linkage means to said armature to provide a spring force component biasing said an'nature toward a normal position with an air gap between said armature and said yoke; said magnetic fluxgenerating a flux force in opposition to said spring force component; said armature, as it moves toward said yoke in a closing stroke, moving said linkage means to operate said spring means in a manner such that said spring force component includes a nonlinear positive spring gradient which closely follows, for a substantial portion of the closing stroke, the flux force gradient of said flux generated by minimum current in said conductor means required to operate said armature toward said yoke; said linkage means also operating said spring means in a manner such that for another portion of said closing stroke near the end thereof, said spring force component does not increase as rapidly as said flux force increases.

2. A device as set forth in claim 1, also including a dashpot opposing movement of said armature in said closing stroke.

3. A device as set forth in claim 1, in which said linkage means comprises first and second means pivotally joined at a knee; said spring means acting at the region of said knee in a direction to collapse said toggle.

4. A device as set forth in claim 1, in which said linkage means includes first and second lever arms keyed together at a common pivot; said first arm being shorter than said second arm; said spring means connected to said first arm at the end thereof remote from said pivot; means providing a lost motion connection between said armature and said second arm at the end thereof remote from said pivot.

5. A device as set forth in claim 4, also including a dashpot opposing movement of said armature in said closing stroke. 

1. An electromagnetically operated device including a magnetic frame and a conductor means coupled thereto so that electric current flow in said conductor means generates magnetic flux in said frame; said frame including a relatively stationary yoke and a relatively movable armature, linkage means, and spring means connected through said linkage means to said armature to provide a spring force component biasing said armature toward a normal position with an air gap between said armature and said yoke; said magnetic flux generating a flux force in opposition to said spring force component; said armature, as it moves toward said yoke in a closing stroke, moving said linkage means to operate said spring means in a manner such that said spring force component includes a nonlinear positive spring gradient which closely follows, for a substantial portion of the closing stroke, the flux force gradient of said flux generated by minimum current in said conductor means required to operate said armature toward said yoke; said linkage means also operating said spring means in a manner such that for another portion of said closing stroke near the end thereof, said spring force component does not increase as rapidly as said flux force increases.
 2. A device as set forth in claIm 1, also including a dashpot opposing movement of said armature in said closing stroke.
 3. A device as set forth in claim 1, in which said linkage means comprises first and second means pivotally joined at a knee; said spring means acting at the region of said knee in a direction to collapse said toggle.
 4. A device as set forth in claim 1, in which said linkage means includes first and second lever arms keyed together at a common pivot; said first arm being shorter than said second arm; said spring means connected to said first arm at the end thereof remote from said pivot; means providing a lost motion connection between said armature and said second arm at the end thereof remote from said pivot.
 5. A device as set forth in claim 4, also including a dashpot opposing movement of said armature in said closing stroke. 